Do not add parentheses when completing functions in closures without parameters

[SomeoneToIgnore]

fn multiply_by_2(input: u32) -> u32 {
    input * 2
}

fn main() {
    let initial = Some(22);
    let multiplied = initial.map(mul<|>);
}

Do a completion.

Expected:

fn multiply_by_2(input: u32) -> u32 {
    input * 2
}

fn main() {
    let initial = Some(22);
    let multiplied = initial.map(multiply_by_2<|>);
}

Actual:

fn multiply_by_2(input: u32) -> u32 {
    input * 2
}

fn main() {
    let initial = Some(22);
    let multiplied = initial.map(multiply_by_2(<|>));
}

[Veykril]

cc #8676 which as context for why this is difficult for us